The present invention is directed to a method of determining the channel impulse response (CIR) of a communication system, such as the CIR of radio channels of a digital mobile radio network (GSM network). In particular, the present invention relates to determining the CIR based on the reception of a known training sequence.
In order to determine CIR, a portion of the transmit signal must be known. For a GSM network, Synchronisation bursts (SB) are a useful portion of the signal. The SB are transmitted on at least one channel from every base station, and they are transmitted in a regular pattern. Decoding of the GSM protocols is not necessary. Both the data in the SB and the pattern with which they occur is fixed and substantially identical for all base stations.
The advantage of using SB for determining the CIR is that they represent a relatively long, noise-like predetermined transmitted signal. Typically, 64 bits are transmitted over a period of 237 xcexcsec. The determination of the CIR thus requires sufficient synchronisation to the bursts in the received signal to enable the extraction of the SB which is then processed to determine the CIR.
The CIR is determined by using estimation techniques. In order to estimate the CIR, a known training sequence is transmitted Stx(t), and this is corrupted by a communications channel producing the received signal Srx(t). The problem in estimating the CIR is to determine tap-weights {xcex1} of a FIR filter (which is used to approximate the CIR), so that the known Stx(t) after passing through the filter is as close as possible to the received signal Srx(t).
Given the that the transmitted Signal Stx(t) and the received signal Srx(t) is known, the CIR can be estimated from:
1. known samples of the transmitted signal Tk=Stx(t0+kxcfx84), xe2x88x92Ncxe2x89xa6k less than N (note that N+Nc samples represent the whole training sequence, the numbering has been arranged to start from the known clean samples of the received signal which occur Nc samples after the start of the training sequence.), and
2. measured samples of the received signal Rk=Srx(to+kxcfx84), 0xe2x89xa6k greater than N (the first Nc samples of the training sequence are assumed corrupted and are ignored).
The tap weights {xcex1k} are determined by simple correlation as                               α          k                =                                            ∑                              j                =                0                                            N                -                1                                      ⁢                                                            T                  _                                                  j                  -                  k                                            ⁢              R              ⁢                              xe2x80x83                            ⁢              j0                                ≤          k          ≤                      N            c                                              (        2.1        )            
This algorithm relies on the noise-like properties of the transmitted signal whereby its autocorrection function should have low time sidelobes. The sidelobe performance of this algorithm, however, has been found to be limited due to the correlation properties of the xe2x80x9ccleanxe2x80x9d part of the training sequence and the fact that only partial correlations are performed for later weights.
The results of using the algorithm to determine the CIR of a simple channel is shown in FIG. 1. The results indicate a dynamic range of approximately 17 dB and a main lobe width of about 9 xcexcsec at the xe2x88x9210 dB point.
It has been found that the correlation technique is extremely robust in noise, however as can be seen from the illustration of FIG. 1, the resolution is poor (wide main beam) and the dynamic range is limited due to relatively high side lobes.
The present invention seeks to alleviate the problems experienced in determining CIR with prior art techniques.
Therefore, the invention discloses a method of estimating the channel impulse response (CIR) of a communication system, the method including the steps of:
(a) providing a first estimation of the CIR by an impulse response calculation using correlation of signals to determine tap weights for said calculation,
(b) determining the true path delay of a radio channel represented by the significant peaks in said first estimation, and
(c) calculating a refined estimation of the CIR using said significant peaks, such that said peaks represent substantially all the energy represented by said first estimation of CIR.
According to another aspect the present invention provides a method of estimating CIR, the method including the steps of
(a) providing a first estimation of the CIR by calculation using correlation,
(b) identifying a first peak having least attenuation in said first estimation,
(c) determining the true path delay represented by the first peak from said first estimation,
(d) calculating a refined estimation of the CIR, from an initial zero refined estimation, by providing a first refined peak in the refined estimation having a path delay corresponding to the true path delay represented by the first peak,
(e) subtracting from the first estimation components corresponding to said first peak;
(f) repeating steps (b) to (e), adding further peaks to the refined estimation until substantially all the energy in said first estimate has been subtracted.
The invention further discloses a device adapted to estimate the CIR of a digital communication system, the device comprising:
first estimating means for determining a first CIR estimation, including impulse response filter means and correlation means for calculating tap weights for said filter means, and
processing means for determining the true path delay represented by the significant peaks in said first estimation, and calculating a refined estimation of the CIR using said significant peaks, such that said peaks represent substantially all the energy represented by said first estimation of CIR.
The present invention also provides a device adapted to estimate the CIR of a digital communication system, the device comprising:
first estimating means for determining a first CIR estimation based on correlation,
first memory means for storing said first CIR estimation,
second memory means for storing a refined estimation of CIR, and
processing means for identifying a first peak having least attenuation in said first estimation, determining the true path delay represented by the first peak from said first estimation so as to define a refined peak, subtracting the correlation components associated with said first peak from said first estimation, and adding said refined peak into said second memory means, said processing means being controlled such that successive peaks in said first estimated CIR are identified, so that after successive iterations a refined estimation of CIR is stored in said second memory means.
The device may be implemented in software on a suitable processing device.
The present invention is based on the realization that the resolution and dynamic range of the technique based on simple correlation can be improved by using a newly developed xe2x80x98line removalxe2x80x99 technique. This involves taking the CIR as determined in the simple correlation, and producing a refined CIR in which the refined CIR is determined by locating the true path delay represented by the peaks resultant from the simple correlation.
This allows for the effects of the less noise-like aspects of the known signal for correlation, that is the larger than desired side lobes, to be minimised or removed from the CIR estimate. The most significant features of the initial correlation are progressively removed from the initial correlation by interpolating their true path delay, adding these peaks to a refined estimation, and subtracting the corresponding correlation components from the initial estimation.